By Stephanie Kelly NEW YORK (Reuters) -Oil tumbled over 3% on Tuesday, retreating from a seven-year high after Russia said some of its military units were returning to their bases after exercises near Ukraine, a move that appeared to de-escalate tension between Moscow and the West. It was not clear how many units were being withdrawn, and by what distance, after a build-up of an estimated 130,000 Russian troops. An earlier Interfax report on the troop movements had prompted oil to extend losses. “The situation is very fluid, but today is definitely a calmer day,” said Robert Yawger, executive …